Universal material testing method and device therefor

ABSTRACT

A load increasing in a stepped manner from an initial load is applied to a test piece ( 4 ) of material in order to examine mechanical properties of the material by applied load, but the load having a next step load added thereto is applied to the test piece after it is confirmed that the variation of strain within a unit time applied to the test piece in the load (Wn) applied in each step falls within a predetermined value whereby there are measured physical variation and time variation in the load applied in each step. The load applied to the test piece is preferably kept at a constant value and the applied load (Wn) and and time variation are preferably recorded.

TECHNICAL FIELD

This invention relates to a universal material test method used forevaluating physical properties of an objective material by loadingtension, compression, bending etc. to a test piece of the objectivematerial and an apparatus for carrying out the test method.

BACKGROUND OF THE INVENTION

Describing a tension tester as an example of a test machine used for theuniversal material test, this tension tester 1 is shown in FIG. 1 andcomprises a base 10, a gate-shaped support frame 11 disposed on an upperface of the base 10 and opened on its front and rear sides, upper andlower chucks 12 and 13 provided at upper and lower positions of the base10, respectively to grapes the test piece 4 and a load mechanism 2 fortension load provided on one of the chucks, which is the upper chuck 12in the illustrated example through a load cell 14, for example. Theother chuck, which is the lower chuck 13 in the illustrated example, issecurely attached to the base 10.

The load mechanism for tension load comprises a servo motor 21 disposedbelow the base 10, two ball screws 18 contained in the support frame 11on both sides there and rotated by the servo motor 21 and a crosshead 17meshed with the ball screws 18 and moved in upward and downwarddirections by rotation of the ball screws 18. The upper chuck 12 issuspended from and supported by the crosshead 17 through the load cell14. Thus, as the crosshead 17 vertically moves in the upward anddownward directions, tension load is applied to the test piece 4 and theactual tension load applied to the test piece 4 is detected byinformation from the load cell 14.

In a conventional material test method using such an apparatus, thetension load is applied to the test piece with predetermined speed (acrosshead speed of 0.1 mm/min., for example) until it reaches the loadassumed to reach a break point of the test material (500 N through 300KN, for example) and variation in strain

while the tension load is being applied to the test material issubsequently measured to obtain a yield point and the break point of thetest material, for example.

The technique of testing the material by applying the predetermined loadto the test material at the suitably set up speed is a generally commonone in the measurement test for specifying the mechanical properties ofthe material such as the tension test, the compression test and thebending test and continues from former times to present times.

The aforementioned conventional technique can obtain the general strain,the yield point, the break point and so on of the test piece, but itsprincipal object is that the load such as tension, compression etc. iscontinuously applied to the test piece at the preset speed until theobjective phenomena (the break, for example) appears and there has beennot noticed variation in the true strain occurring in the test piecewhen certain load is applied thereto. In other words, for example, inthe measurement of the yield point where plastic deformation of thematerial starts, there appears the phenomena that the strain is slowlyadvancing in long time under the fixed load and therefore in theconventional measurement depending on the tension speed, the measurementis made in the state where the strain is still advancing and in anunstable condition. Thus, the next load is applied to the test piecebefore the strain beginning to occur at certain load point stops, whichcauses the problem of the reliability of the measurement value and thebasis thereof.

Therefore, there are some cases where the same material would havevarious strain speeds depending on the set tension speed (the speedcorresponding to the movement speed of the crosshead, in theaforementioned example) and would have different strain speed dependingon the applied load. In the conventional test method having such aviewpoint not aimed, there is a problem in which the true mechanicalproperties of the material have not been obtained.

Since the test apparatus for carrying out the aforementionedconventional test method has not been designed from such a viewpoint, anoperator can work only paying attention to what speed at which the loadshould be applied to the test piece, how correctly the strain should bemeasured or how correctly the yield point and the break point should bemeasured. Even though the computer technology, which has accomplished aremarkable development in recent years, is used, the precision and theautomation of the measurement have been just asked for without havinggotten out of the conventional measurement viewpoint.

The object of the invention is to provide a universal material testmethod and a test apparatus suitable for carrying out this test methodadapted to be able to judge the property or the treatment process of amaterial by measuring a time in which the strain of the material getsstable under a new viewpoint that strain of the material varies onchange of the component of the material and change of organization ofthe material when it is treated and especially thermally treated whenconstant load is applied to the material, which fundamentally changesfrom the point view of the prior art material test method.

DISCLOSURE OF THE INVENTION

This invention provides a universal material test method and anapparatus therefor having the following features in order to accomplishthe aforementioned object.

More particularly, the universal material test method of the inventionis the method of testing mechanical properties of an objective materialby measuring a change in the load by applying tension, compression,bending etc. to a test piece of the objective material in which the loadincreasing from a set-up initial load in a stepped manner is applied tothe test piece and characterized in that the load having a next stepload added thereto is applied to the test piece after it is confirmedthat the variation of the strain within a unit time applied to the testpiece in the load applied in each step falls within a predeterminedvalue whereby physical variation and time variation in the load appliedin each step is measured.

An apparatus for carrying out the test method of the invention ischaracterized by comprising a loading mechanism (2) to apply a load to atest piece (4), detection means (5) to detect a deformation of the testpiece (4), judgment means (J) to judge that a variation of strain of thetest piece within the unit time when the load is applied falls within apredetermined value on the information from the detection means (5) andload control means to instruct the loading mechanism to further add astep load of predetermined value after the judgment means (J) judgesthat the variation of the strain of the test piece within the unit timefalls within the predetermined value.

There may be used the loading mechanism (2), which can apply the stepload to the test piece in the stepped manner. For instance, there may beused means to sequentially accumulate a weight of predetermined load,but in order to mechanically practice this, there may be desirably usedconstant load control means (C) to maintain a uniform value of the loadapplied to the test piece (4).

The apparatus of the invention may comprise record means to record theload (Wn) of each step and the physical variation and the time variationthereof for the purpose of analyzing the measured data later.

The test method of the invention continues to apply the load (Wn) ofconstant weight by suspending the weight of predetermined unit valuefrom a lower end of the test piece until the variation of the strainfalls within the predetermined value and sequentially apply the load(Wn) having another step load added thereto when it is confirmed thatthe variation of the strain falls within the predetermined value, whichhas a procedure substantially different from that of the prior testmethod and therefore, the inventor calls this “dead weighted loadingtype universal test method” or “dead weighted loading type universaltest machine” whereby it can be distinguished from an oil pressure type,screw type or operating lever type universal test machine of the priorart.

The reference numbers having a parenthesis attached thereto in theclaims and the column of means to resolve the problems are convenientlyadded thereto for more easily understanding the construction of theinvention and it should be noted that the construction of the inventionis not limited to the form illustrated in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a brief front view of a universal material test apparatusconstructed in accordance with one embodiment of the invention;

FIG. 2 is a flow chart showing a control of a universal material testmethod of one embodiment of the invention;

FIG. 3 illustrates a graph showing values measured by the universalmaterial test method according to the embodiment of the invention; and

FIG. 4 illustrates a graph showing values measured by the universalmaterial test method according to the embodiment of the invention.

BEST MODE OF CARRYING OUT THE INVENTION

A concrete form of embodiment of the invention will be described andillustrated with reference to the drawings hereinafter.

The test apparatus of the invention can be used as universal materialtest machine such as a tension test machine, a compression test machine,a bending test machine, etc, but in the present embodiment, it will beexplained as the tensile test machine.

The tensile test machine 1 according to the present embodiment may havethe construction similar to that of the test machine described withreference to the prior test method and comprises a base 10, a supportframe 11, upper and lower chucks 12 and 13, a load cell 14, a crosshead17, ball screws 18 and a loading mechanism 2.

Onto the support frame 11 are disposed and attached an indicator toindicate initial setting information for arranging the mount of the testpiece 4 on the upper and lower chucks 12 and 13 and an operation box 16to perform the operation thereof. The load mechanism 2 comprises speedchange means 20 to upwardly and downwardly move the upper chuck 12 in avertical direction through the association with the crosshead 17 and theball screws 18 and a servo motor 21 to supply a fine drive power to thespeed change means 20. The servo motor 21 performs “constant loadcontrol” to maintain the set load at a constant value and “applied loadcontrol” to apply the load having a set step load (ΔW) added thereto.Each of the controls may be performed by a program control using apersonal computer 3 and sequential information of the step load, theapplied load (Wn), the lapse time etc., are displayed on a displayinstrument 31 and recorded in record means such as hard disk, flexibledisk, etc.

The constant load control means C to perform the “constant load control”among the aforementioned controls is for controlling the set load (100N, for example) to be always applied to the lower end of the test piece4. In the load action using the crosshead 17, for example, since anelongation occurs when the test piece 4 is initially tensioned by theload of 100 N and the initial load (100 N) varies, a signal from theload cell 14 is monitored on real time and the load servo motor 21 iscontrolled to be finely rotated so that the set load (100 N) is alwaysapplied to the test piece 4. This means that the same state as the statewhere the weight is suspended from the lower end of the test piece 4 isaccomplished by the mechanical tension of the upper chuck 12. Themechanism itself to accomplish the constant load control means C is theconventional technique.

The applied load control means L to accomplish the “applied loadcontrol” among the aforementioned controls is for controlling the load(Wn) having the predetermined step load (ΔW) sequentially added to theset initial load (W1) therefrom to act to the test piece 4. This isperformed in accordance with the control flow chart described later.

A strain gauge 5 is disposed between the objective points of the testpiece 4 grasped by the upper and lower chucks 12 and 13 and a signalobtained by amplifying the strain

of the test piece 4 by a strain meter 32 is fed to the personal computer3 performing the controls on real time.

The test method of the embodiment of the invention is realized bycontrolling the test machine 1 constructed in the aforementioned manneras described later. This will be explained hereinafter on the flow chartshown in FIG. 2.

At first, in the STEP (briefly referred to as just “S” hereinafter) 1, acommand to apply to the test piece 4 the initial load W1 to start themeasurement is fed to the servomotor 21. The initial load W1 isappropriately set within the resilient range of the test piece 4. Asensor signal from the load cell 14 is monitored so as to be able tomaintain the constant initial load W1 (see S2) and the aforementionedservo motor 21 is finely driven in a proper manner in consideration of avariation in the tensile load due to the delicate elongation whereby theload during its application is always kept at a constant value (see S3).

Thereafter, the applied load reaches the aforementioned initial load(W1) and the measurement of strain

starts on the output signal from the strain gauge 5 mounted on the testpiece 4 while the constant load is maintained by the constant loadcontrol means C (see S4). Subsequently, the strain

when the measurement starts is once read and recorded (see S5), thestrain

is again read and recorded (see S7) after predetermined time (10 second,20 second and so on, for example) lapses (see S6) and whether adifference

between the previous and last strains (=

b−

a) falls within the predetermined allowance range P (|

d|≦P) or not is judged by the judgment means J (see S8). If thedifference between the previous and last strains is out of thepredetermined allowance range P (|

d|>P), the procedure is returned to the STEP 5 and the measurement ofthe strain

a (see S5), the measurement of the strain

b (see S7) after the predetermined time (10 second, 20 second and so on,for example) lapses (see S6) and the arithmetical operation of thedifference

d between the previous and last strains are performed and repeated untilthe difference

d falls within the predetermined allowance range P(|

d |≦P).

If the difference

d falls within the predetermined allowance range P(|

d|≦P), it is judged that the advance of the strain in the initial load(W1) substantially stops (see S9) and the lapse time and so on at thattime are recorded in the record means (internal or external memorydevices). Although the predetermined allowance range P is desirablyzero, the tension of the test piece 4 is applied by the aforementionedload mechanism 2 and therefore a little deflection occurs. Thus, waitinguntil the difference

d reaches zero is not realistic and therefore the difference

d converging within the predetermined allowance value P is deemed thatthe strain advance stops. In the case where the load is applied bysuspending the weight, but not by the aforementioned load mechanism 2,the stop of the advance of the strain (P=0) may be judged.

After |

d|≦P is judged, the applied load control means L feeds to the servomotor 21 the load command of the applied load W2 (=W1+ΔW) for the secondstep having the predetermined step load ΔW added to the initial load W1(see S10).

Herein, the second step applied load W2 is again maintained at theconstant value by the aforementioned constant load control until theadvance of the strain stops (including the fictitious stop). When theadvance of the strain due to the second step applied load W2 stops,another step load ΔW is added and the operation is repeated. This is thesame system as the predetermined weight is suspended from the lower endof the test piece 4 and another weight is added and suspended when theadvance of strain stops. The load by the weight is maintained in thestate where the constant applied load is always maintained during theapplication thereof. The aforementioned constant load control means C isused for accomplishing this state by the aforementioned tensionmechanism.

In this manner, the next stage applied load Wn+1 (=Wn+ΔW) having thestep load ΔW added to the applied load Wn while maintained at theconstant value is sequentially applied to the test piece 4 in thestepped manner whereby the strain in each step and the time in which theadvance of the strain stops (including the fictitious stop) can bemeasured.

EXAMPLE OF MEASUREMENT VALUE

The graphized value measured using the aforementioned test method is asshown in FIGS. 3 and 4.

FIG. 3 shows how the strain varies when the next stage applied load(Wn+1) having the step load ΔW added to the initial load is applied tothe test piece and the time Δt in which the step load (ΔW) is applied,the variation of the strain immediately after the step load (ΔW) isapplied (the next stage applied load (Wn+1) is applied), the time (tn)in which the strain is stabilized and the variation

n of the strain which occurs until the difference stably falls withinthe allowance value P immediately after the next stage applied load isapplied are shown in FIG. 3.

FIG. 4 is a graph showing the relationship between stress and strain inthe measurement by the test method of the embodiment of the inventionand the prior test method. It will be noted from the graph that thereappears no phenomena in which the stress slightly decreases at the yieldpoint y, which appears in the prior test method and it will be notedthat the strain continues to increase in a natural manner immediatelybefore the test piece is broken.

Although, in the aforementioned embodiment, the strain is measured bythe strain gauge 5 adhered to the test piece 4, the strain measurementmeans is not limited thereto, of course, and there may be used either ofcontact type and non-contact type sensor means.

With the invention constructed as aforementioned, the following effectscan be obtained. When the load is applied to metal material, deformationcorresponding to the load never occurs at moment, but it will occur in acertain time and the time required for the deformation depends onmaterials. Thus, it will be noted that the peculiar action (therelationship between the stress and the time) of the materials can beexamined by measuring this time. Since these actions correspond to thevariation in the component of the materials, the organization due totheir heat treatment and so on, the properties of the materials can beexamined by measuring the time for their deformation.

The following can be seen by measuring the time constant. When thestress-strain curve is prepared, it can be expressed so that thesingular points (upper yield point and lower yield point) of thephenomena in which the stress slightly decreases at the yield point asshown in the prior art never appear and the strain smoothly increasesimmediately before the test piece is broken. When the test piece towhich the load is once applied nearly until it is broken is again testedin the similar manner, the variation in the strain is different from theprevious test.

Furthermore, the component of the materials and the difference due tothe method of manufacture can be quantified and the change of the stepsof manufacture and the change of the manufacture lots even for thematerials of the same component ratio cause the variation in the strainspeed, for instance. Thus, the change of the steps of manufacture andthe change of the manufacture lots can be judged by patternizing thesevariations in the strain speed and comparing these patterns.

UTILIZABILITY FOR INDUSTRIES

As aforementioned, the method of the invention can properly evaluate theproperties of the objective materials and judging the variation in thestep of manufacture and the manufacture lots using the results of theevaluation.

1. A universal material test method for examining mechanical propertiesof an objective material by measuring deformation with a change in aload applied to a test piece of said objective material in which theload increasing from a set-up initial load in a stepped manner isapplied to said test piece and characterized in that the load having anext step load of preselected constant value added thereto is applied tothe test piece after it is confirmed that the variation of strain withina unit time resulting from a prior step load of preselected constantvalue applied to the test piece is stabilized at a predetermined valuein each step whereby there are measured physical variation and timevariation and the load applied in each step.
 2. A universal materialtest apparatus characterized by comprising a loading mechanism to applya load to a test piece, detection means to detect a deformation of thetest piece, judgment means to judge that a variation of strain of thetest piece within a unit time when the load is applied falls within apredetermined value based on information from said detection means andload control means to instruct the loading mechanism to further add astep load of preselected contant value after said judgment means judgesthat the variation of the strain of the test piece within the unit timeis stabilized within the predetermined value.
 3. A universal materialtest apparatus as set forth in claim 2, and further comprising recordmeans to record the load applied in each step, its physical variationand time variation until said variation of strain is stabilized withinthe predetermined value in each step.
 4. A universal material testapparatus as set forth in claim 1, further including the step ofexamining the measured physical variation, time variation and loadapplied in each step to evaluate historical properties of the saidobjective material.